This invention relates to an oscillator and, more particularly, to an oscillator for an oscillating signal synchronous with an input signal.
A monitor display is usually connected to a personal computer. A video signal is supplied to the monitor display, and carries pictures at synchronizing frequencies depending upon the number of pixels defined in a VGA (Variable Graphic Array) or an SVGA (Super Variable Graphic Array), by way of example. Even though the video signal is supplied to the monitor display at different synchronizing frequencies, it is necessary that the picture size and the location of the picture are stable on the screen of the monitor display. For this reason, a multi-sink monitor display is widely used for the video signal supplied at different frequencies.
A synchronous signal processing circuit is integrated on a semiconductor chip, and the semiconductor integrated circuit device is incorporated in the multi-sink monitor display. The synchronous signal processing circuit achieves an automatic regulation for the horizontal output frequency, and the automatic horizontal frequency regulation circuitry is responsive to variation of the horizontal input frequency so as to make the free running oscillating frequency follow the horizontal input frequency. The phase and the frequency of the free running oscillating signal are coincident with those of the horizontal input signal.
FIG. 1 illustrates the prior art automatic horizontal frequency regulation circuitry. The prior art automatic horizontal frequency regulation circuitry comprises a counter 51, a digital-to-analog converter 52, a voltage-controlled oscillator 53 and a phase comparator 54. The counter 51 is connected to the digital-to-analog converter 52, which in turn is connected to the voltage-controlled oscillator 53. The voltage-controlled oscillator 53 and a signal input node are connected to the phase comparator, and the phase comparator 54 regulates the oscillation frequency through the comparison between the input signal at the input node 101 and the free-running oscillation signal 503.
The prior art automatic horizontal frequency regulation circuitry behaves as follows. FIG. 2 shows the relation between the input frequency and a control voltage signal 502. The digital-to-analog converter 52 linearly increases the magnitude of the control voltage signal 502 together with the input frequency, and the counter 51 and the digital-to-analog converter 52 uniquely determines the magnitude of the control voltage signal 502 on the basis of the input frequency. On the other hand, the relation between the control voltage signal 502 and the free-running oscillation signal 503 is shown in FIG. 3. The frequency f0 is linearly increased together with the magnitude of the control voltage signal 502. Thus, the voltage-controlled oscillator 52 uniquely determines the free-running oscillation frequency on the basis of the magnitude of the control voltage signal. When the gradient of the plots is appropriately regulated, the voltage-controlled oscillator 53 varies the frequency of the free-running oscillation signal 503 in such a manner as to become coincident with the input frequency.
A problem is encountered in the prior art automatic horizontal frequency regulation circuitry in that a post regulation is required after the completion of the fabrication process. In detail, the voltage-controlled oscillator 53 repeats the charge into capacitors and the discharge therefrom for oscillating the free-running signal. The circuit components of the prior art automatic horizontal frequency regulation circuitry are integrated on a semiconductor chip through the fabrication process, and the capacitors are also formed on the semiconductor chip. However, the capacitance is liable to be dispersed, and the relation between the magnitude of the control voltage signal and the free-running frequency is not constant among the products. For example, if the voltage-controlled oscillator 53 is designed as indicated by real line in FIG. 4, the voltage-controlled oscillator 53 of a product may have the voltage-to-frequency characteristics indicated by broken line. In this instance, the free-running frequency is f02 at the control voltage signal v1, and is different from the designed frequency f01. When the difference exceeds the control range of the loop consisting of the voltage-controlled oscillator 53 and the phase comparator 54, the prior art automatic horizontal frequency regulation circuitry can not output any in-phase signal. In order to rescue the defective products from rejection, an external regulation circuit is required for the prior art automatic horizontal frequency regulation circuitry, and the manufacturer carries out the post regulation by using the external regulation circuit. The external regulation circuit makes the prior art synchronous signal processing circuit large, and increases the production cost thereof. Although discrete capacitors may avoid the problems, the semiconductor integrated circuit device requires additional pins to be connected to the discrete capacitors, and the discrete capacitors are not feasible.
It is therefore an important object of the present invention to provide an oscillation circuit, which automatically synchronizes an output oscillation signal with an input signal without any additional regulation circuit.
To accomplish the object, the present invention proposes to vary the control range of a voltage-controlled oscillator in the vicinity of the frequency of an input signal.
In accordance with one aspect of the present invention, there is provided a oscillating circuit for producing an output signal synchronous with an input signal comprising a signal input node supplied with the input signal, a voltage-controlled oscillator having voltage-to-frequency characteristics achieved in a certain control range and responsive to a control voltage signal for changing a frequency of the output signal, a first control loop connected to the signal input node and the voltage-controlled oscillator so as to compare the frequency of the output signal with an frequency of the input signal to see whether or not the input signal falls within the certain control range and changing a first sub-signal of the control signal so as to make the input signal fall within the certain control range when the input signal is out of the certain control range, and a second control loop connected to the signal input node and the voltage-controlled oscillator and controlling the voltage-controlled oscillator with a second sub-signal of the control signal so as to make the output signal and the input signal in-phase.